* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

html, body {
    height: 100%;
    /* 位置是相对于 common.css 来说，cat.jpg 在 当前目录(css/)的父目录(..)的img目录下 */
    background-image: url("../img/cat.jpg");
    background-position: center center;
    background-size: cover;
    background-repeat: no-repeat;
}

.nav {
    width: 100%;
    height: 50px;
    background-color: rgba(51, 51, 51, .4);
    color: #fff;

    display: flex;
    justify-content: left;
    align-items: center;
}

.nav img {
    /* 规定图片大小时，尽量只高或者宽，这样，图片不会被拉伸变形 */
    height: 40px;
    margin-left: 30px;
    margin-right: 10px;
    border-radius: 50%;
}

.nav .spacer {
    width: 70%;
}

.nav a {
    color: #fff;
    text-decoration: none;
    padding: 0 10px;
}

.container {
    height: calc(100% - 50px);
    width: 1200px;
    margin: 0 auto;
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.container-left {
    height: 100%;
    width: 300px;
}

.container-right {
    height: 100%;
    width: 895px;
    /* 当内容超过外部容器的高度时，怎么办？ auto：没超过时正常显示，超过时，出现滚动条 */
    overflow: auto;
    background-color: rgba(255, 255, 255, .8);
    border-radius: 10px;
}

.card {
    background-color: rgba(255, 255, 255, .8);
    border-radius: 10px;
    padding: 30px;
}

.card .avatar {
    margin-left: 50px;
    width: 140px;
    border-radius: 50%;
}

.card h3 {
    text-align: center;
    padding: 30px;
}

.card a {
    display: block;
    text-align: center;
    color: #999;
    text-decoration: none;
    padding: 10px;
}

.card .counter {
    padding: 5px;
    display: flex;
    justify-content: space-around;
}